Fitting of brightness in a visual prosthesis

ABSTRACT

The invention is a method of automatically adjusting an electrode array to the neural characteristics of an individual patient. The perceptual response to electrical neural stimulation varies from patient to patient and The response to electrical neural stimulation varies from patient to patient and the relationship between current and perceived brightness is often non-linear. It is necessary to determine this relationship to fit the prosthesis settings for each patient. It is advantageous to map the perceptual responses to stimuli. The method of mapping of the present invention is to provide a plurality of stimuli that vary in current, voltage, pulse duration, frequency, or some other dimension; measuring and recording the response to those stimuli; deriving a formula or equation describing the map from the individual points; storing the formula; and using that formula to map future stimulation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application60/653,674, filed Feb. 16, 2005, for A Method of Determining theElectrical Current Amplitude Required to Produce a Percept. Thisapplication is related to and incorporates herein by reference, U.S.patent application Ser. No. 10/864,590 filed Jun. 8, 2004 for AutomaticFitting for a Visual Prosthesis.

GOVERNMENT RIGHTS NOTICE

This invention was made with government support under grant No.R24EY12893-01, awarded by the National Institutes of Health. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention is generally directed to neural stimulation andmore specifically to an improved method of Optimizing neural stimulationlevels for artificial vision.

BACKGROUND OF THE INVENTION

In 1755 LeRoy passed the discharge of a Leyden jar through the orbit ofa man who was blind from cataract and the patient saw “flames passingrapidly downwards.” Ever since, there has been a fascination withelectrically elicited visual perception. The general concept ofelectrical stimulation of retinal cells to produce these flashes oflight or phosphenes has been known for quite some time. Based on thesegeneral principles, some early attempts at devising a prosthesis foraiding the visually impaired have included attaching electrodes to thehead or eyelids of patients. While some of these early attempts met withsome limited success, these early prosthetic devices were large, bulkyand could not produce adequate simulated vision to truly aid thevisually impaired.

In the early 1930's, Foerster investigated the effect of electricallystimulating the exposed occipital pole of one cerebral hemisphere. Hefound that, when a point at the extreme occipital pole was stimulated,the patient perceived a small spot of light directly in front andmotionless (a phosphene). Subsequently, Brindley and Lewin (1968)thoroughly studied electrical stimulation of the human occipital(visual) cortex. By varying the stimulation parameters, theseinvestigators described in detail the location of the phosphenesproduced relative to the specific region of the occipital cortexstimulated. These experiments demonstrated: (1) the consistent shape andposition of phosphenes; (2) that increased stimulation pulse durationmade phosphenes brighter; and (3) that there was no detectableinteraction between neighboring electrodes which were as close as 2.4 mmapart.

As intraocular surgical techniques have advanced, it has become possibleto apply stimulation on small groups and even on individual retinalcells to generate focused phosphenes through devices implanted withinthe eye itself. This has sparked renewed interest in developing methodsand apparatuses to aid the visually impaired. Specifically, great efforthas been expended in the area of intraocular retinal prosthesis devicesin an effort to restore vision in cases where blindness is caused byphotoreceptor degenerative retinal diseases such as retinitis pigmentosaand age related macular degeneration which affect millions of peopleworldwide.

Neural tissue can be artificially stimulated and activated by prostheticdevices that pass pulses of electrical current through electrodes onsuch a device. The passage of current causes changes in electricalpotentials across visual neuronal membranes, which can initiate visualneuron action potentials, which are the means of information transfer inthe nervous system.

Based on this mechanism, it is possible to input information into thenervous system by coding the information as a sequence of electricalpulses which are relayed to the nervous system via the prostheticdevice. In this way, it is possible to provide artificial sensationsincluding vision.

One typical application of neural tissue stimulation is in therehabilitation of the blind. Some forms of blindness involve selectiveloss of the light sensitive transducers of the retina. Other retinalneurons remain viable, however, and may be activated in the mannerdescribed above by placement of a prosthetic electrode device on theinner (toward the vitreous) retinal surface (epiretial). This placementmust be mechanically stable, minimize the distance between the deviceelectrodes and the visual neurons, and avoid undue compression of thevisual neurons.

In 1986, Bullara (U.S. Pat. No. 4,573,481) patented an electrodeassembly for surgical implantation on a nerve. The matrix was siliconewith embedded iridium electrodes. The assembly fit around a nerve tostimulate it.

Dawson and Radtke stimulated cat's retina by direct electricalstimulation of the retinal ganglion cell layer. These experimentersplaced nine and then fourteen electrodes upon the inner retinal layer(i.e., primarily the ganglion cell layer) of two cats. Their experimentssuggested that electrical stimulation of the retina with 30 to 100 uAcurrent resulted in visual cortical responses. These experiments werecarried out with needle-shaped electrodes that penetrated the surface ofthe retina (see also U.S. Pat. No. 4,628,933 to Michelson).

The Michelson '933 apparatus includes an array of photosensitive deviceson its surface that are connected to a plurality of electrodespositioned on the opposite surface of the device to stimulate theretina. These electrodes are disposed to form an array similar to a “bedof nails” having conductors which impinge directly on the retina tostimulate the retinal cells. U.S. Pat. No. 4,837,049 to Byers describesspike electrodes for neural stimulation. Each spike electrode piercesneural tissue for better electrical contact. U.S. Pat. No. 5,215,088 toNorman describes an array of spike electrodes for cortical stimulation.Each spike pierces cortical tissue for better electrical contact.

The art of implanting an intraocular prosthetic device to electricallystimulate the retina was advanced with the introduction of retinal tacksin retinal surgery. De Juan, et al. at Duke University Eye Centerinserted retinal tacks into retinas in an effort to reattach retinasthat had detached from the underlying choroid, which is the source ofblood supply for the outer retina and thus the photoreceptors. See,e.g., E. de Juan, et al., 99 Am. J. Ophthalmol. 272 (1985). Theseretinal tacks have proved to be biocompatible and remain embedded in theretina, and choroid/sclera, effectively pinning the retina against thechoroid and the posterior aspects of the globe. Retinal tacks are oneway to attach a retinal array to the retina. U.S. Pat. No. 5,109,844 tode Juan describes a flat electrode array placed against the retina forvisual stimulation. U.S. Pat. No. 5,935,155 to Humayun describes aretinal prosthesis for use with the flat retinal array described in deJuan.

In addition to the electrode arrays described above, there are severalmethods of mapping a high resolution camera image to a lower resolutionelectrode array. U.S. Pat. No. 6,400,989 to Eckmiller describesspatio-temporal filters for controlling patterns of stimulation in anarray of electrodes. The assignee of the present application has tworelated U.S. patent applications: Ser. No. 09/515,373, filed Feb. 29,2000, entitled Retinal Color Prosthesis for Color Sight Restoration; andSer. No. 09/851,268, filed May 7, 2001, entitled Method, Apparatus andSystem for Improved Electronic Acuity and Perceived Resolution Using EyeJitter Like Motion. Both applications are incorporated herein byreference.

Each person's response to neural stimulation differs. In the case ofretinal stimulation, a person's response varies from one region of theretina to another. In general, the retina is more sensitive closer tothe fovea. Any stimulation, with magnitude less than the threshold ofperception, is ineffective. Stimulation beyond a maximum level will bepainful and possibly dangerous to the patient. It is therefore,important to map any video image to a range between the minimum andmaximum for each individual electrode. With a simple retinal prosthesis,it is possible to adjust the stimulation manually by stimulating andquestioning the patient. As resolution increases, it is tedious orimpossible to adjust each electrode by stimulating and eliciting apatient response.

A manual method of fitting or adjusting the stimulation levels of anauditory prosthesis is described in U.S. Pat. No. 4,577,642, Hochmair etal. Hochmair adjusts the auditory prosthesis by having a user compare areceived signal with a visual representation of that signal.

A more automated system of adjusting an auditory prosthesis using middleear reflex and evoked potentials is described in U.S. Pat. No.6,157,861, Faltys et al. An alternate method of adjusting an auditoryprosthesis using the stapedius muscle is described in U.S. Pat. No.6,205,360, Carter et al. A third alternative using myogenic evokedresponse is disclosed in U.S. Pat. No. 6,415,185, Maltan.

U.S. Pat. No. 6,208,894, Schulman describes a network of neuralstimulators and recorders implanted throughout the body communicatingwirelessly with a central control unit. U.S. Pat. No. 6,522,928,Whitehurst, describes an improvement on the system described in Schulmanusing function electro stimulation also know as adaptive deltamodulation to communicate between the implanted devices and the centralcontrol unit.

The greatest dynamic range is achieved by setting the minimumstimulation at the threshold of perception and the maximum stimulationlevel approaching the pain threshold. It is unpleasant for a patient tofirst concentrate to detect the minimum perception and then be subjectedto stimulation near the threshold of pain.

The human retina includes about four million individual photoreceptors.An effective visual prosthesis may include thousands of electrodes. Anautomated system is needed to adjust individual electrodes in a visualprosthesis for maximum benefit without the need for patient interactionin a long and difficult process.

SUMMARY OF THE INVENTION

The invention is a method of automatically adjusting an electrode arrayto the neural characteristics of an individual patient. The response toelectrical neural stimulation varies from patient to patient and therelationship between current and perceived brightness is oftennon-linear. It is necessary to determine this relationship to fit theprosthesis settings for each patient. It is advantageous to map theperceptual responses to stimuli. The method of mapping of the presentinvention is to provide a plurality of stimuli that vary in current,voltage, pulse duration, frequency, or some other dimension; measuringand recording the perceptual response to those stimuli; deriving aformula or equation describing the map from the individual points;storing the formula; and using that formula to map future stimulation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the implanted portion of the preferredretinal prosthesis.

FIG. 2 a-d are graphs showing typical current vs. brightness response.

FIG.3 is a flowchart show the brightness mapping method.

FIG. 4 is a flow chart showing an alternate process of auto fitting anelectrode array.

FIG. 5 depicts a block diagram of the retinal prosthesis electroniccontrol unit.

FIG. 6 is a graph depicting a typical neural response to electricalinput.

FIG. 7 depicts an alternate fitting process using cortical recording.

FIG. 8 depicts an alternate fitting process using iris recording.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims.

FIG. 1 shows a perspective view of the implanted portion of thepreferred retinal prosthesis. A flexible circuit 1 includes a flexiblecircuit electrode array 10 which is mounted by a retinal tack (notshown) or similar means to the epiretinal surface. The flexible circuitelectrode array 10 is electrically coupled by a flexible circuit cable12, which pierces the sclera and is electrically coupled to anelectronics package 14, external to the sclera.

The electronics package 14 is electrically coupled to a secondaryinductive coil 16. Preferably the secondary inductive coil 16 is madefrom wound wire. Alternatively, the secondary inductive coil 16 may bemade from a flexible circuit polymer sandwich with wire traces depositedbetween layers of flexible circuit polymer. The electronics package 14and secondary inductive coil 16 are held together by a molded body 18.The molded body 18 may also include suture tabs 20. The molded body 18narrows to form a strap 22 which surrounds the sclera and holds themolded body 18, secondary inductive coil 16, and electronics package 14in place. The molded body 18, suture tabs 20 and strap 22 are preferablyan integrated unit made of silicone elastomer. Silicone elastomer can beformed in a pre-curved shape to match the curvature of a typical sclera.However, silicone remains flexible enough to accommodate implantationand to adapt to variations in the curvature of an individual sclera. Thesecondary inductive coil 16 and molded body 18 are preferably ovalshaped. A strap 22 can better support an oval shaped coil.

The preferred prosthesis includes an external portion (not shown) whichincludes a camera, video processing circuitry and an external coil forsending power and stimulation data to the implanted portion.

FIGS. 2 a-d show typical perceptual responses collected from fourpatients. The perceptual responses differ in both the amplitude of theresponse curve and the shape of the response curve. All four patientperceptual responses, however, can be fitted by the function B=aI^(b)where B is brightness, I is current amplitude, and a and b areparameters to be estimated from fitting the empirical data. Three datapoints will adequately define the function. Numerous statistical toolsare available for automatically fitting the function to the three datapoints.

In this example the x axis represent the amplitude of stimulation usinga single pulse. The y axis represents the patient's subjective rating ofbrightness where a stimulus rated as “10” is twice as bright as astimulus rated as “5”.

FIG. 3 shows a flow chart of the fitting procedure. In this case, we areusing patient's ratings of subjective brightness but a measure of neuralactively such a neural recording or pupil response (described below)could be used in an analogous fashion. First the fitting system mustdetermine the perceptual brightness response to current relationship.This is accomplished by stimulating and measuring the subject reportedbrightness response rating at three points. It should be noted that theresponse is near linear in most cases. Hence, two points can be used toapproximate the response, but three points will yield a more accuratefit. First, a stimulus is presented 23. If there is no response 24, thestimulus is increase 25 and stimulation is presented again 23. If thereis a response to stimulus 24 and the response is pain 26, thestimulation is reduced 27 and stimulation is presented again 23. Ifthere is a non-painful response it is recorded 28 in in non-volatilememory of the prosthesis device. Recording the response may includesubjective response, neural recording or other physiological response.This process is repeated to get the required number of recordedresponses, usually 3. If there are three recorded responses 29, anequation or formula is derived to describe the relationship betweencurrent and brightness relationship 30. The formula may be saved as anactual equation to be applied to the input value, or as a table of inputand output values. It should also be noted that there must be a maximumcharge limit sent in a visual prosthesis for safety reasons. Hence, thecurrent variations must be limited by the preprogrammed maximum change.In the preferred embodiment, current is mapped to brightness. It shouldbe noted that other a factors which may affect brightness, such asvoltage, pulse width or frequency, may be mapped by the same method.

After the formula is established, input is received by the camera 31;the formula is applied to input data 32; and an output value is used tostimulate neural tissue 33.

FIG. 4 is a flow chart of an automatic fitting sequence which may beemployed to gain the three points needed for the method described inFIG. 3, or may be used as an alternative fitting procedure. In the flowchart, the value N is the selected electrode, X is the neural activityrecorded, and L is the level of stimulation (current amplitude. First Nis set to 0 40 and then incremented 42. The first electrode, electrodeN, is addressed 44. The stimulation level is set to zero 46, and thenincremented 48. The neural tissue is stimulated at the minimum level 50.The stimulation is immediately followed by a recording of activity inthe neural tissue 52. Alternatively, recording can be donesimultaneously by an adjacent electrode. If recording is donesimultaneously, one must distinguish between neural activity andelectrical charge from the stimulating electrode. The neural responsefollows stimulation (see FIG. 6). Simultaneous stimulation and recordingrequires that the recording phase be longer than the stimulation phase.If so, the stimulation and neural response can be separated digitally.If the recorded neural activity is less than a predetermined level 54,the stimulation level is increased and steps 48-54 are repeated.

In most cases, the preset minimum level is any measurable neuralactivity. However, perception by the patient is the determining factor.If neural activity is detected and the patient reports no perception,the minimum level must be set higher. Once minimum neural activity isrecorded, the stimulation level is saved in memory 56. The level is thenfurther increased 58 and stimulation is repeated 60. Again stimulationis immediately followed by recording neural activity 62. If apredetermined maximum level has not been reached, steps 58-64 arerepeated until the predetermined maximum stimulation level is obtained.Once the predetermined maximum stimulation level is obtained, steps42-64 are repeated for the next electrode. The process is continueduntil a minimum and maximum stimulation level is determined for eachelectrode 66.

To obtain the subjective brightness or neural response for the necessarythree points, one first finds the stimulus amplitude (the intensity ofthe stimulus can also be varied along other dimensions) which is barelydetectable by the patient or provokes a minimally detectable neuralresponse. One then presents the stimulus at that value (e.g. theamplitude value V=42) repeatedly until one has an accurate measurementof the subjective brightness or neural response at that stimulusintensity. One then finds the stimulus amplitude that is just under thesafety limit or pain threshold, and measures the apparent brightness orneural response at that stimulus intensity. Finally one finds apparentbrightness or neural response for a stimulus whose amplitude is halfway(or intermediate) between those two points. If additional data pointsare desired, equal distant points such as 25% and 75% should be used.

The range of intensities used for stimulation during operation of thedevice will fall within the range that is measured during the fittingprocedure. Very low or high intensity values may not be used in normalfunction/.

The maximum stimulation level borders on discomfort for the patient.Because the automatic fitting process is automated, high levels ofstimulation are only applied for a few microseconds. This significantlydecreases the level of discomfort for the patient compared withstimulating long enough to elicit a response from the patient.

The fitting process is described above as an incremental process. Thefitting process may be expedited by more efficient patterns. For examplechanges may be made in large steps if it the detected response issignificantly below the desired response, followed by increasingly smallsteps as the desired response draws near. The system can jump above andbelow the desired response dividing the change by half with each step.

Often, neural response in a retina is based, in part, on geographicalcloseness. That is, neurons closer to the fovea require less stimulationthan neurons farther from the fovea. Hence once a stimulation is levelis set for an electrode, one can presume that the level will be similarfor an adjacent electrode. The fitting process may be expedited bystarting at a level near the level set for a previously fit adjacentelectrode.

Automating the fitting process has many advantages. It greatly expeditesthe process reducing the efforts of the patient and clinician. Further,the automated process based on measured neural responses is objective.Patient perceptual responses are subjective and may change over time dueto fatigue. In some cases, a patent may not be able to provide therequired responses due to age, disposition, and/or limited metalability.

FIG. 5 depicts a block diagram of the control unit. The block diagram isa functional diagram. Many of the functional units would be implementedin a microprocessor. A control unit 80 sets and increments a counter 82to control the stimulation level of the stimulator 84. The stimulationsignal is multiplexed in MUX 86 to address individual electrodes 88.After each stimulation, the addressed electrode returns a neuralactivity signal to a recorder 90. The signal is compared to the storedminimum or maximum level (stored in a memory 92) in a comparator 94.After programming, a signal from a video source 96, or other neuralstimulation source, is adjusted in a mapping unit 98, in accordance withthe minimum and maximum levels stored in the memory 92. The adjustedsignal is sent to the stimulator 84, which in synchronization with MUX86 applies the signal to the electrodes 88. The electronics for thecontrol unit could be external or within the implanted prosthesis.

FIG. 6 is a graphical representation of the neural response toelectrical stimulus. This figure is derived from actual recordings of afrog retina. Response in a human retina will be similar and can bemeasured by a retinal electrode, implanted cortical electrode, orexternal cortical electrode commonly known as a visual evoked responseor VEP. The vertical axis is current while the horizontal axis is time.Four curves 100-106 show the response at varying input current levels.An input pulse 108, is followed by a brief delay 110, and a neuralresponse 112. Hence, it is important to properly time the detectingfunction. Either the stimulating electrode must be switched to adetecting electrode during the brief delay or detecting must occur onanother electrode and continue long enough to record the neuralresponse. It should also be noted that the delay period 110 becomesshorter with increased stimulation current. Hence, the system mustswitch faster from stimulation mode to detecting mode with increasedcurrent. The change in delay time may also be used as an additionalindication of neural response. That is, the minimum and maximum may bedetermined by matching predetermined delay times rather thanpredetermined output levels. As stimulation increases, it becomes moreuseful to employ an alternate recording means as described in thefollowing alternate embodiments.

In a first alternate embodiment, the recording electrode may be corticalelectrode mounted on or near the visual cortex. Temporary externalelectrodes placed on the scalp proximate to the visual cortex may recordneural activity in the visual cortex. This allows the system to accountfor any variations in neural processing between the retina and thevisual cortex. It, however, requires electrodes either implanted in thevisual cortex or placed temporarily near the visual cortex. Thisalternate embodiment may be combined with the preferred embodiment byfirst using cortical electrodes to perform an initial fitting of theprosthesis in a clinic. Thereafter, retinal recording may be used toreadjust the prosthesis for any changes over time.

FIG. 7 shows the first alternate retinal prosthesis. A stimulatingelectrode array 150 is placed against the outer surface of a retina 152(epiretinally). A cable 154 pierces a sclera 156 and attaches to anelectronic control unit 158. A return electrode 160 may be placeddistant from the retina 152. The stimulating electrode array 150 is aplurality of tiny electrodes. One or more recording electrodes 162 areplaced in near the visual cortex. The recording electrodes may betemporary external electrodes, implanted electrodes under the scalp, orelectrode implanted within the visual cortex.

In a second alternate embodiment, the recording electrode may be eitherimplanted in the iris, or placed externally near the iris. The iriscontracts when increasing light levels enter the eye. Electricalstimulation of the retina also causes the iris to contract, because thebody perceives an increase in light entering the eye. Conversely, theiris expands in response to a decrease in electrical stimulation. Whilethe response of the iris is relatively slow, the neurological signalsinitiating a change in the iris respond quickly. Measuring these signalsmay provide alternate feed back as to the body's response to theelectrical stimulus. Alternatively, an optical device aimed at the eyemay detect the diameter of the iris.

FIG. 8 shows the second alternate retinal prosthesis. A stimulatingelectrode array 210 is placed against the outer surface of a retina 212(epiretinally). A cable 214 pierces a sclera 216 and attaches to anelectronic control unit 218. A return electrode 220 may be placeddistant from the retina 212. The stimulating electrode array 210 is aplurality of tiny electrodes. A recording electrode 224 is place in theperiphery of the iris sensing electrical stimulus to the iris.

In a third alternate device, electroluminescent pigments may be appliedto the retina. Electroluminescent pigments cause an individual cell toglow when it fires an action potential. A camera of the type used forretinal photos may detect neural response by detecting theelectroluminescent glow of the applied pigment.

Accordingly, what has been shown is an improved method of stimulatingneural tissue for improved response to brightness. While the inventionhas been described by means of specific embodiments and applicationsthereof, it is understood that numerous modifications and variationscould be made thereto by those skilled in the art without departing fromthe spirit and scope of the invention. It is therefore to be understoodthat within the scope of the claims, the invention may be practicedotherwise than as specifically described herein.

1. A method of fitting a visual prosthesis, comprising: applying aplurality of stimuli to visual neural tissue; measuring brightnessresponses to said stimuli to determine a threshold of perception;applying an additional stimuli higher than said threshold of perception:measuring a brightness response to said additional stimuli; deriving aformula comprising a stimulation curve estimating brightness levels nottested, said formula being derived starting from empirical data of saidthreshold of perception and said brightness response to said additionalstimuli and corresponding to a curve fitting of the empirical data; andgenerating said stimuli by applying said formula to visual input.
 2. Themethod according to claim 1, wherein said stimuli vary according toapplied current.
 3. The method according to claim 1, wherein saidstimuli vary according to applied voltage.
 4. The method according toclaim 1, wherein said stimuli vary according to applied frequency. 5.The method according to claim 1, wherein said stimuli vary according toapplied pulse width.
 6. The method according to claim 1, wherein saidbrightness responses are perceptions of brightness reported by a user.7. The method according to claim 1, wherein said responses are neuralresponses recorded from the visual neural tissue of the user.
 8. Themethod according to claim 1, wherein said responses are recordedphysiological changes in the iris of a user.
 9. The method according toclaim 1, wherein said responses are recorded neural activity of theuser.
 10. The method according to claim 1, further comprising creating atable based on said formula.
 11. The method according to claim 10,wherein said step of generating stimuli by applying said formulacomprises applying values from said table.
 12. The method according toclaim 1, wherein said formula defines brightness as slope times currentto the power of shape where slope and shape are derived terms.
 13. Themethod according to claim 1, wherein said formula defines brightness asslope times current plus threshold, where slope and threshold arederived terms.
 14. The method according to claim 1, wherein said formulais a functionB=a I^(b), wherein B represents brightness, I represents currentamplitude, and a and b represent parameters estimated from fitting theempirical data.
 15. A visual prosthesis, comprising: an array ofelectrodes suitable to be placed in proximity to visual neural tissue; acamera for receiving a visual scene; and a control device coupling saidcamera to said array of electrodes including a neural stimulator forapplying a plurality of stimuli to visual neural tissue; means formeasuring brightness responses to said stimuli for determining athreshold of perception; means for applying an additional stimuli higherthan said threshold of perception; means for measuring a brightnessresponse to said additional stimuli; means for deriving a formulacomprising a stimulation curve estimating brightness levels not tested,said formula being derived starting from empirical data of saidthreshold of perception and said brightness response to said additionalstimuli and corresponding to a curve fitting of the empirical data; andmeans for generating stimuli by applying said formula to visual inputfrom said camera.
 16. The visual prosthesis according to claim 15,wherein said means for measuring brightness responses is a means formeasuring neural activity in the retina.
 17. The visual prosthesisaccording to claim 15, wherein said means for measuring brightnessresponses is a means for measuring neural activity in the visual cortex.18. The visual prosthesis according to claim 15, wherein said means formeasuring brightness responses is a means for measuring a patient'sreported responses.
 19. The visual prosthesis according to claim 15,wherein said means for measuring brightness responses is a means formeasuring movement of the iris.
 20. The visual prosthesis according toclaim 15, wherein said means for generating stimuli varies by appliedcurrent.
 21. The visual prosthesis according to claim 15, wherein saidmeans for generating stimuli varies by applied voltage.
 22. The visualprosthesis according to claim 15, wherein said means for generatingstimuli varies by applied frequency.
 23. The visual prosthesis accordingto claim 15, wherein said means for generating stimuli varies by appliedpulse duration.
 24. The visual prosthesis according to claim 15, whereinsaid formula is a functionB=a I^(b), wherein B represents brightness, I represents currentamplitude, and a and b represent parameters estimated from fitting theempirical data.